Cross‐sectional images showed the initial attachment of B. fragilis to the mucin layer of the μGut, followed by the formation of multicellular biofilms after 24 h (Figure 3d and Video S6, Supporting Information), consistent with biofilms previously observed in inflammatory bowel disease (IBD) patients[40] and other animal studies.[41] The initial binding of B. fragilis was found to be spatially localized near the crypt of the μGut (12–17 μm in height) (Figure 3e). Here, MUC5AC is linked to inflammatory bowel disease.